Device for Concentrating a Liquid, and Differential Piston Pump

ABSTRACT

Disclosed is a device (V) for concentrating a liquid (F) which is pumped through a concentrating apparatus ( 3 ) by means of a differential piston pump and directly impinges the differential piston (K) in an assisting manner during the conveying stroke at a residual pressure as a concentrate (FK). The differential piston of said device (V) is driven by a drive shaft ( 12 ) that actuates a linearly guided valve control member ( 18 ) of an intake and discharge valve assembly (A). The differential piston pump is embodied as a radial piston pump (R) in which eccentric attachments ( 13, 14 ) for the differential piston (K) and the valve control member ( 18 ) are disposed on the drive shaft ( 12 ), the differential piston (K) and the valve control member ( 18 ) being hinged directly to the eccentric attachments ( 13, 14 ) by means of traction and pressure couplings (S).

The invention relates to a device of the type specified in the preamble of patent claim 1 and to a differential piston pump as per the preamble of patent claim 2.

When concentrating liquids, for example in the desalination of sea water (reverse osmosis) or in the production of fruit juice, on account of the process, a large amount of primary energy is consumed for pumping the liquid, with the concentrate which is generated being accumulated with a relatively high residual pressure. In the case of the desalination of sea water in particular, the high demand for primary energy has long been a known disadvantage which, in the past, and with regard to the high energy costs, has for example been minimized in that the energy which is contained in the concentrate on account of the residual pressure is used in order to assist the feeding action of the differential piston pump, and to thereby save on primary energy.

In the device for sea water desalination known from EP 0 450 257 B1, and in the differential piston pump shown in EP 0 450 257 B1, FIG. 5, the differential piston, with the side remote from the piston rod, pumps the liquid which is to be concentrated, for example sea water, through inlet and outlet valves in a manner controlled by means of the diaphragms which are for example designed to operate according to the principle of reverse osmosis and downstream of which accumulate concentrate with high residual pressure and permeate. The concentrate which is under the residual pressure is introduced cyclically, in a manner controlled by the inlet and outlet valve arrangement, into the piston rod side of the differential piston, in order to directly assist the pumping action by means of the residual pressure before the concentrate is discharged substantially unpressurized. The linearly guided shaft of the differential piston is coupled by means of a connecting rod to a crank pin of the drive shaft which is embodied as a crankshaft. The inlet and outlet valve arrangement of the pressure chamber on the piston rod side of the differential piston has separate inlet and outlet valves which are actuated, counter to a spring force, by means of linearly guided plungers of control cams which are arranged on the drive shaft.

Since the springs are strong and progressive, they generate an undesired power loss. The crankshaft/connecting-rod arrangement leads not only to a large installation size of the differential piston pump but also causes, for example on account of the connecting rod deflection, a piston stroke movement with low harmonic properties, which results in further power losses and pulsations in the pumped liquid.

The invention is based on the object of specifying a device of the type specified in the introduction which can be operated with a minimal use of primary energy, and a compact and extremely efficient differential piston pump.

The set object is achieved according to the invention by means of the features of patent claim 1 or patent claim 2.

In the device, the radial piston pump concept results in a small installation size, in a low-pulsation operating behavior and, on account of the pull and push couplings, in a high level of efficiency with minimal losses, so that very little primary energy is sufficient, and the energy in the concentrate is utilized optimally and above all directly. The pull and push couplings lead to an extremely harmonic stroke profile of the differential piston and of the valve control member. The control of the inlet and outlet valves to the pressure chamber takes place in a manner optimally adapted to the stroke profile of the differential piston, and with minimal wear over a long service life. The radial piston pump concept makes it possible, in the case of sea water desalination, to create and use compact devices in a manner so to speak tailored in terms of dimensions for the demands of individual buildings or groups of buildings, with it being possible, on account of the low primary energy use, even for direct current primary energy from solar panels to deliver high throughput rates for example in sunny and coastal regions. Compact devices of said type can therefore be used without a mains electricity connection or even in regions in which an electricity supply is not or rarely available or would be too expensive for domestic use.

The differential piston pump having the radial piston concept without a connecting rod and above all without energy-sapping strong valve springs is extremely suitable, on account of the compact installation size and the high level of efficiency, not only for sea water desalination but for all applications in which a liquid is to be pumped and the same liquid or the concentrated liquid or some other liquid with an appreciable residual pressure is available.

In one expedient embodiment, a single valve control member is provided for in each case one inlet and one outlet valve in order to minimize energy losses. On account of the push and pull coupling to the eccentric on the drive shaft, energy-sapping valve springs are dispensed with, which promotes efficiency and the service life, since the inlet and outlet valve arrangement operates with relatively low forces and contains no components which are susceptible to damage.

In one expedient embodiment, a draw body mounted rotatably on the eccentric is provided, to which the shaft end is coupled in a form-fitting manner, so that the rotational movement of the eccentric is transmitted in a harmonic manner. The slideway permits the transverse offset movements between the rotating eccentric and the linearly movable shaft without appreciable wear and without noises and vibrations.

The slideway, which acts between the rotating eccentric and the linearly guided shaft, expediently provides protection against rotation for the shaft. Alternatively, the slideway can also be designed such that the shaft can rotate for the distribution of wear.

The draw body is expediently formed, for assembly reasons, either from assembled halves or even in one piece, and such that it has for the shaft an externally open pocket which, for the form-fitting action, is narrower than the external width of the shaft. The form-fitting interaction takes place between faces which can be dimensioned to be relatively large, which prevents local wear.

In order to prevent transverse loads on the shaft in operation, it can be advantageous to arrange the shaft in a slide guide fixed to the housing. The slide guide can be an inserted sliding bushing. In addition, the slide guide with formed-on supporting shells should engage with a sliding fit into the pocket in order to support the shaft over the greatest possible length.

Here, it is expedient for assembly reasons if either the shaft or the supporting shells or both position the draw body axially on the eccentric.

A bottom surface of the pocket should form a pressure surface for the shaft end (push coupling), which surface is, on account of the required sliding movement, longer than the external width of the shaft end. At a distance from the shaft end, parallel grooves are formed in the shaft, which grooves define a width approximately corresponding to the internal width of the pocket, and at the shaft end, form externally protruding drivers as an engaging member (on one or on both sides of the shaft) which engages in undercuts of the pocket (pull coupling). The sliding movement between the shaft end and the draw body takes place optimally close to the eccentric, which promotes smooth running.

In one expedient embodiment, the undercuts in the pocket extend parallel to a tangent to the eccentric, with the undercuts being longer in this direction than the driver so as not to hinder the relative sliding movement. The undercuts can be of continuous design, which can simplify the assembly of the shaft end in the draw body, so that the draw body can if appropriate be formed in one piece.

In order that the piston rod side of the differential piston is acted on over the delivery stroke by the residual pressure in a precisely controlled manner, while the residual pressure is promptly dissipated during the return stroke of the differential piston, it is important that the eccentric for the valve control member of the outlet and inlet valve arrangement assigned to a differential piston is offset relative to the eccentric for this differential piston by 90° about the axis of the drive shaft and in a leading manner in the driving direction of rotation.

With regard to low-noise and low-pulsation running, it is expedient for three or more differential pistons to be uniformly distributed in a stellate arrangement about the axis of the drive shaft and to provide a common eccentric and a common draw body for these differential pistons. In this way, the moving masses are reduced, and installation space is saved. It is possible to arrange a plurality of such differential piston groups along the drive shaft.

The valve control members for the outlet and inlet valve arrangements of the plurality of differential pistons are expediently also distributed regularly in a stellate arrangement about the axis of the drive shaft, wherein a common eccentric and a common draw body should be provided for the valve control members for structural reasons.

In one expedient embodiment, the outlet and inlet valve arrangement comprise, fixed to the housing, an inlet valve seat and an outlet valve seat, which are oriented radially relative to the axis of the drive shaft and are coaxial, with the outlet valve seat being positioned closer to the drive shaft than the inlet valve seat. As a result of said positioning, the region in which high residual pressure acts for a relatively long time is as remote as possible from the drive shaft, and the sealing between the inlet valve seat and the drive shaft is scarcely subjected to the high residual pressure or is subjected to the high residual pressure in each case only for a short time. The arrangement of the valve seats takes up little installation space in the direction of the axis of the drive shaft.

The valve control member should extend with sufficient play through the outlet valve seat as far as into the inlet valve seat, with a sealing region being provided between the valve seats and also with respect to the drive shaft. First and second shoulders on the valve control member serve for lifting the respective valve bodies from their seats, with the lifting movements of the valve bodies taking place in opposite directions. That is to say that the valve control member, as it lifts, opens the inlet valve, and in contrast, as it falls, opens the outlet valve.

In an expedient manner in terms of assembly, the shoulders are formed by the ends of a tube fixed, preferably screwed, to the valve control member, in order to obtain contact regions with the largest area possible.

The valve bodies are acted on in the direction of their seats by means of only weak springs, with the inlet valve body being a disk or a plate which can be centered by the spring. The outlet valve body, in contrast, is an annular disk or a circular-ring-shaped plate which is guided on the valve control member and can move relative thereto. Here, a sliding ring seal is expediently provided between the valve control element and the outlet valve body in order to avoid leakage, which bypasses the outlet valve body, in the outlet valve seat.

With regard to correct sealing conditions when the valve bodies are seated, conical or spherical sealing faces could be provided on the valve bodies, and the valve seats could also be formed with conical or spherical sealing faces.

In order to ensure that no overlap occurs between the outlet and inlet valves, that is to say each valve opens only once the other valve has closed, it is important for the distance in the longitudinal direction of the valve control member between the shoulders to be smaller than the distance between the seated valve bodies. Here, the dimensioning is preferably carried out in such a way that, at the top and bottom dead centers of the differential piston, both valve bodies are seated and play is present relative to the respective shoulder of the valve control member. Said play can for example be between approx. 0.1 and 0.4 mm.

The valve control member is expediently guided at multiple points over its length.

The valve seats and a guide for the valve control member can be arranged in sleeve members, which are mounted in a housing chamber between a housing wall adjacent to the drive shaft and a housing cover. Said sleeve members can comprise an inlet and two outlets, while a further inlet is arranged in the wall of the housing chamber. This concept simplifies assembly and makes it possible, in order to save on installation space, to position the inlet and outlet valves close to one another.

In one expedient embodiment, the eccentrics for the differential piston and the valve control member are constructed with identical dimensions and with identical eccentricity.

In one particularly expedient embodiment, the eccentricity of the eccentric relative to the axis of the drive shaft can be adjusted in order to be able to adapt the delivery capacity and/or the operating behavior on demand. This could be realized for example in that the eccentric is composed of two eccentric sleeves which are inserted one into the other and which are rotatable relative to one another and can fix any desired relative positions. Alternatively, the eccentric could also be arranged replaceably on the drive shaft, so that a change in the eccentricity can be brought about by replacing the eccentric.

Embodiments of the subject matter of the invention are explained on the basis of the drawing, in which:

FIG. 1 shows a device for concentrating a pumped liquid, in a schematic illustration, having a radial piston pump which is shown in a partial axial section, with a differential piston being at top dead center,

FIG. 2 shows the radial piston pump during the suction stroke,

FIG. 3 shows the radial piston pump during the delivery stroke,

FIG. 4 shows a cross section of a design detail of the radial piston pump in the section plane IV-IV of FIG. 5,

FIG. 5 shows an axial section of the detail of FIG. 4, in the section plane V-V,

FIG. 6 schematically shows a further design detail, and

FIG. 7 shows a further design detail.

A device V, shown schematically in FIG. 1, for concentrating a liquid F into a concentrate or a concentrated liquid F_(K), for example while simultaneously forming pure liquid F_(R), could be a sea water desalination plant which operates according to the principle of reverse osmosis, or a plant for concentrating fruit juice or the like. It is fundamentally provided in the device V to feed and pressurize the liquid F by means of a radial piston pump R, to conduct said liquid F through a concentrating means 3, out of which the concentrated liquid F_(K) is accumulated under a considerable residual pressure, while the pure liquid F_(R) is collected in a substantially unpressurized state, for example as pure water permeate in the case of sea water desalination. The concentrated liquid F_(K) with the considerable residual pressure is used in the radial piston pump R in order, by means of the contained energy, to directly assist the delivery of the liquid F in order that only a small amount of primary energy, for example electrical current or the power of a motor P, is consumed for the operation of the radial piston pump R. In the case of sea water desalination, the concentrating means 3 would for example be a diaphragm system which operates according to the principle of reverse osmosis.

The radial piston pump R in FIG. 1 sucks the liquid F, which is if appropriate provided with a slight pilot pressure, via a line 1 and a supply valve 16 by means of at least one differential piston K (suction stroke), and during the delivery stroke, feeds said liquid F via a discharge valve 17 into a line 2. The line 2 leads to the concentrating means 3, out of which the pure liquid F_(R) emerges, and in a line 4, the concentrated liquid F_(K) is fed under the residual pressure to an inlet 5. After the concentrated liquid F_(K) has been used in a pressure chamber 36 for acting on the differential piston K during the delivery stroke, said liquid flows, substantially unpressurized, out of an outlet 6.

In order to simplify the starting of the device, it is possible for a valve device 7 to be provided in the line 2, which valve device 7 feeds via a line 8 directly into the inlet 5, but in normal operation is scarcely used or is used only under certain circumstances.

The radial piston pump R has a housing wall 10, which separates the delivery and working region from a chamber 11 of a drive shaft 12, and for example a removable housing cover 9.

Arranged on the drive shaft 12 are an eccentric 13 for driving the differential piston K and a further eccentric 14 for actuating an inlet and outlet valve arrangement A, with the eccentric 14 being offset in the driving direction of rotation 40 with respect to the eccentric 13 by approximately 90° about an axis 38 of the drive shaft 12. In the embodiment shown, the two eccentrics 13 are constructed with different dimensions and are arranged on the drive shaft 12 with different eccentricities. It would however be entirely possible for the two eccentrics 13, 14 to be constructed with identical dimensions and to also be arranged with identical eccentricities. The eccentrics 13, 14 can be fixedly formed on the drive shaft 12 or can be replaceably wedged thereon. In an alternative which is not shown, the eccentricity of each eccentric with respect to the axis 38 of the drive shaft 12 can be varied, for example by means of a replacement or by virtue of each eccentric 13 or 14 being composed of two eccentric sleeves which are rotatable relative to one another and can be fixed in selectable relative positions. In FIG. 1, the axis of the eccentric 13 is denoted by 37 and the axis of the eccentric 14 is denoted by 39.

The differential piston K has, on the piston rod side, a shaft 15 which is connected by means of a pull-push coupling directly to the eccentric 13, is linearly guided in the housing wall directly or indirectly at 37, and is sealed off. The differential piston K comprises a sealing arrangement which separates a pump chamber from the pressure chamber 36.

The inlet and outlet valve arrangement A comprises an inlet valve composed of a valve body 32 and a valve seat 27, and an outlet valve composed of a valve body 29 and a valve seat 28. The valve seats 27, 28 are aligned radially with respect to the axis 38 of the drive shaft 12 and are coaxial, with the outlet valve seat 28 pointing toward the drive shaft 12 and being arranged closer to the latter than the inlet valve seat 27 which points away from the drive shaft 12. Assigned to the two valves is a common valve control member 18 which is linearly guided directly or indirectly in the housing wall 10, and is sealed off, at at least one point, and is directly articulatedly connected to the eccentric 14 by means of a push and pull coupling S, with a shaft 19 of the valve control member 18 extending with radial play from the coupling S through the outlet valve seat 28 as far as into the inlet valve seat 27. The seals 29 seal between the outlet valve and the chamber 11 and between the outlet 6 and the inlet valve seat 27. In addition, a further guide 21 for the valve control member 18 is provided between the valve seats. A tube 22 is fixed, for example screwed, to a stepped shaft section of the valve control member 18, which tube 22 forms a first shoulder 23 which points toward the outlet valve body 29 and a second shoulder 24 which points toward the inlet valve body 32. The distance between the shoulders 23, 24 is smaller than the distance between the valve bodies 29, 32 when the latter are positioned on their valve seats 27, 28, in such a way that, when the valves are closed, there is for example a play of between 0.1 and approximately 0.4 mm between each shoulder 23 or 24 and the adjacent valve body 32 or 29. The inlet valve body 32 can be a disk or a plate and is centered and loaded in the direction of the inlet valve seat 27 by means of a weak spring 33. The valve body 29 can be a circular-ring-shaped disk or a circular-ring-shaped plate which is loaded by means of a spring 31 toward the outlet valve seat 28. A sliding ring seal 30 can be provided between the outlet valve body 29 and the shaft 19 of the valve control member 18. The valve bodies 32, 29 can have conical or rounded seat faces, as can the valve seats 27, 29. The inlet valve seat 27 can be formed in a sleeve part 25, while the guide 21 and the seal 29 and the outlet valve seat 28 can be formed in a further sleeve member 26. The sleeve members 25, 26 are mounted in the housing between the housing wall 10 and the housing cover 9. Downstream of the inlet valve seat 27, an outlet in the sleeve body 26 leads to an inlet 35 to the pressure chamber 36. The inlet 35 is at the same time connected to a chamber, which is situated below the sleeve body 26 and comprises the outlet valve body 29, at the outlet valve seat 28.

Function:

The drive shaft 12 is driven by means of the primary drive source P, for example an electric motor or an internal combustion engine, in order to drive the differential piston K and the valve control member 18 in an oscillating fashion by means of the couplings S. During the suction stroke, the differential piston K sucks in liquid F, by means of the side remote from the piston rod, via the open supply valve 16. During the suction stroke, the inlet valve 32, 27 is closed and the outlet valve 29, 28 is open. The concentrated liquid F_(K) is discharged, substantially unpressurized, out of the pressure chamber 36 through the outlet 6. Shortly before bottom dead center is reached, or generally in the region of bottom dead center, of the differential piston K, the outlet valve 29, 28 is closed and only then, without a valve overlap, is the inlet valve 27, 32 opened. At the start of the delivery stroke, the concentrated liquid F_(K), which is under residual pressure, in the pressure chamber 36 acts on the piston rod side of the differential piston K in order to provide assistive work in the delivery stroke. Shortly before or in the region of top dead center of the differential piston K, the inlet valve 32, 27 is closed again and only then, without a valve overlap, is the outlet valve 29, 28 opened.

A plurality of differential pistons K and also a plurality of inlet and outlet valve arrangements A, for example at least three or more, are expediently arranged around the drive shaft 12 in a stellate and uniformly distributed fashion.

On account of the assistance by the concentrated liquid F_(K) with its residual pressure, the operation of the radial piston pump R requires so little primary energy that the device V for sea water desalination, for example for the drinking water demand of a building or of a plant, can be operated autonomously by means of a direct current motor from solar panels.

The area ratio between the side remote from the piston rod and the piston rod side of the differential piston K is coordinated with the quantity ratios between the liquid which is to be pumped and the concentrated liquid in such a way that the energy in the concentrated liquid can be optimally utilized for assistance. Here, the pressure difference at the sealing device of the differential piston K is relatively small, and the sealing region 20 is also in each case acted on with the residual pressure for only a short time during the delivery stroke. An oil sump can be provided in the housing chamber 11.

FIG. 2 shows the suction stroke of the differential piston K in the radial piston pump R. The concentrated liquid F_(K) in the pressure chamber 36 is expanded via the open outlet valve 28, 29 and is discharged into the outlet 6, while the residual pressure acts in the inlet 5 when the inlet valve 27, 32 is closed. The shoulder 23 holds the outlet valve 28, 29 open, while the shoulder 24 is removed from the inlet valve body 32. At a residual pressure of for example 68 bar in the inlet 5, a pressure only of approximately 1 bar prevails in the pressure chamber 36. A delivery pressure of approximately 70 bar prevails downstream of the closed discharge valve 17, while the suction pressure when the supply valve 16 is open is approximately 1 bar. Approximately the same pressure therefore prevails on both sides of the differential piston K.

FIG. 3 shows the delivery stroke of the radial piston pump R, in which the differential piston K moves in the direction of top dead center. The valve control member 18 has opened the inlet valve 32, 27, while the outlet valve 28, 29 is closed. The concentrated liquid F_(K) flows, with the residual pressure of for example 68 bar, into the pressure chamber 36 and assists the differential piston K. The outlet valve 28, 29 is held closed by means of said pressure. The liquid which is to be pumped is under a pressure of approximately 70 bar, with the supply valve 16 being closed and the discharge valve 17 being open. The pressure difference at the sealing device of the differential piston K is only approximately 2 bar.

FIGS. 4 and 5 show an embodiment of the coupling S for example between the shaft 15 and the eccentric 13. In FIGS. 4 and 5, the guide 37 of FIG. 1 is formed by a sliding bushing 41 in the housing wall 10, which sliding bushing 41 dips with two supporting shells 42 into the chamber 11 and supports and guides the shaft 15 in the rotational direction of the eccentric 13 with respect to transverse forces. The supporting shells 42 engage as far as into a pocket 45 of a draw body 44 which is rotatably mounted on a sliding bearing bushing or a needle-roller bearing 43 on the eccentric 13 and which is for example positioned axially on the eccentric 13 by means of the shaft 15 and/or the supporting shells 42. The pocket 45 has an internal width which approximately corresponds to the external dimension of the supporting shells 42, so that a slight sliding fit is generated here. Formed in the pocket 45 are a lower pressure surface 49 for the shaft end (push coupling) and, in undercuts 46 in the side walls of the pocket 45, tension surfaces 50 (pull coupling) for the shaft end. The shaft end comprises two grooves 47 which are parallel to one another, so that two outwardly extending drivers 48 are formed on the shaft end, which drivers 48 engage into the undercuts 46. The undercuts 46 are longer than the external width of the end of the shaft 15 and extend if appropriate as far as the periphery of the draw body 44.

The draw body 44 can be formed in one piece or can (FIG. 5) be assembled from two halves 44 a and 44 b. The form-fitting engagement between the drivers 48 and the undercuts 46 also provides protection against rotation for the shaft 15. If appropriate, the grooves 47 are combined into one peripheral groove, and the two drivers 48 also form a collar which is round in the peripheral direction, so that the shaft 15 can rotate in the coupling S.

In the simplified embodiment of FIG. 6, a widening is formed on the end of the shaft 15, which widening forms one or two drivers 48′ and engages into the undercut 46′ of the draw body 44. In the transverse direction in FIG. 6, sufficient play is provided between the drivers 48′ and the undercut 46′ and between the shaft 15 and the inlet to the undercut 46′ in order to permit the sliding movement, indicated by means of a double arrow, of the shaft 15 during the rotational movement of the eccentric 13 about the axis 37.

In the embodiment in FIG. 7, bearing blocks 51 are formed on the draw body 44, in which bearing blocks 51 is seated a sliding pin 52 which extends through the end of the shaft 15. Sufficient play is provided between the bearing blocks 51 in order to permit the sliding movement of the shaft 15 in the slide guide, indicated by a double arrow in FIG. 7, during the rotational movement of the eccentric about the axis 37. 

1. A device (V) for concentrating a liquid (F) pumped by a differential piston pump through a concentrating means (3), which liquid acts, as concentrate (FK) under residual pressure, directly on the at least one differential piston (K) in such a manner as to assist the delivery stroke, the differential piston (K) being driven by a drive shaft (12) which actuates at least one linearly guided valve control member (18) of an inlet and outlet valve arrangement (A), wherein the differential piston pump is a radial piston pump (R), in that eccentrics (13, 14) for the differential piston (K) and the valve control member (18) are arranged on the drive shaft (12), and in that the differential piston (K) and the valve control member (18) are coupled directly to the eccentrics (13, 14) with push and pull couplings (S).
 2. A differential piston pump for liquids (F), having a housing comprising an inflow and an outflow for the liquid (F) to be pumped and an inlet (5) and an outlet (6) for liquid (FK) under residual pressure, at least one differential piston (K) delivering the liquid (F) from the inflow into the outflow, which differential piston may be driven by a drive shaft (12) and during the delivery stroke also directly by the residual pressure and separates a pump chamber from a pressure chamber (36), at least one inlet and outlet valve arrangement (A) for the pressure chamber (36), and at least one linearly guided valve control member (18), actuated by the drive shaft (12), of the inlet and outlet valve arrangement (A), wherein the differential piston pump is a radial piston pump (R), in that eccentrics (13, 14) for the differential piston (K) and the valve control member (18) are arranged on the drive shaft (12), and in that the differential piston (K) and the valve control member (18) are coupled directly to the eccentrics (13, 14) with push and pull couplings (S).
 3. The differential piston pump as claimed in claim 2, wherein a single valve control member (18) is provided for the inlet and outlet valve arrangement (A).
 4. The differential piston pump as claimed in claim 2, wherein the push and pull coupling (S) comprises a draw body (44) mounted rotatably on the eccentric (13, 14) and at least one engaging member (48) provided at one end of a shaft (15, 19) of the differential piston (K) or of the valve control member (15), and in that between the engaging member (48) and the draw body (44) there are provided a substantially radially active form-fitting engagement and, in the circumferential direction, preferably parallel to a tangent to the eccentric (13, 14), a slideway.
 5. The differential piston pump as claimed in claim 4, wherein the slideway provides protection against rotation for the shaft (15, 19).
 6. The differential piston pump as claimed in claim 4, wherein the draw body (44), which, preferably, consists of two axially assembled halves (44 a, 44 b) or is in one piece, comprises for the shaft (15, 19) an externally open pocket (45) undercut in the slideway, whose internal width, when viewed in the axial direction, is less than the external width of the shaft (15, 19).
 7. The differential piston pump as claimed in claim 6, wherein the shaft (15, 19) of the differential piston (E) and/or of the valve control member (18) is arranged in a slide guide (37) fixed to the housing, in that the slide guide (37) comprises a sliding bushing (41), and in that the sliding bushing (41) engages with formed-on supporting shells (42), oriented towards one another in the direction of rotation of the eccentric (13, 14), with a sliding fit between side walls of the pocket (45).
 8. The differential piston pump as claimed in claim 6, wherein the draw body (44) is positioned axially on the eccentric (13, 14) via the slideway and/or the supporting shells (42).
 9. The differential piston pump as claimed in claim 6, wherein a bottom of the pocket forms a pressure surface (49) for the shaft end, which surface is longer than the external width of the shaft end, in that, at a distance from the shaft end, two diametrically opposing parallel grooves (45) or a circumferential groove are formed in the shaft, which grooves define a width approximately corresponding to the internal width of the pocket (45), and in that the shaft end adjoining the grooves or groove forms as engaging member (48) at least one externally protruding driver, which engages in an undercut (46) formed in a pocket side wall.
 10. The differential piston pump as claimed in claim 9, wherein the undercut (46) extends parallel to a tangent to the eccentric (13, 14) and is longer in this direction than the driver.
 11. The differential piston pump as claimed in claim 2, wherein the eccentric (14) for the valve control member (18) of the outlet and inlet valve arrangement (A) assigned to a differential piston (K) is offset relative to the eccentric (13) for this differential piston (K) by approximately 90° about the axis (38) of the drive shaft (12) and in a leading manner in the driving direction of rotation (40).
 12. The differential piston pump as claimed in claim 2, wherein three or more differential pistons (K) are distributed in a stellate arrangement in the axis (38) of the drive shaft (12), and in that a common eccentric (13) and a common draw body (44) are assigned to the differential pistons (K).
 13. The differential piston pump as claimed in claim 12, wherein the valve control members (18) for the outlet and inlet valve arrangements (A) of the differential pistons (K) are distributed regularly in a stellate arrangement about the axis (38) of the drive shaft (12), preferably with a respective offset of approximately 90° relative to the differential pistons (K), and in that a common eccentric (14) and a common draw body (44) are assigned to the valve control members (18).
 14. The differential piston pump as claimed in claim 2, wherein the outlet and inlet valve arrangement (A) comprise, fixed to the housing, an inlet valve seat (27) and an outlet valve seat (28), which are oriented radially relative to the axis (38) of the drive shaft (12) and are coaxial, in that the outlet valve seat (28) is positioned closer to the drive shaft (12) than the inlet valve seat (27), and in that the outlet valve seat (28) points towards the drive shaft (12) and the inlet valve seat (27) points away from the drive shaft (12).
 15. The differential piston pump as claimed in claim 14, wherein the valve control member (18) extends in each case with radial spacing through the outlet valve seat (28) as far as into the inlet valve seat (27), is sealed relative to the drive shaft (12) and between the valve seats (27, 28), and comprises a first shoulder (23) for lifting an outlet valve body arranged movably on the side of the outlet valve seat (28) pointing towards the drive shaft (12) and a second shoulder (24) for lifting an inlet valve body arranged movably on the side of the inlet valve seat pointing away from the drive shaft (12).
 16. The differential piston pump as claimed in claim 15, wherein the shoulders (23, 24) are formed by the ends of a tube (22) fixed, preferably screwed, to the valve control member (18).
 17. The differential piston pump as claimed in claim 15, wherein the inlet valve body (32) is a disk loaded by a spring (33) towards the inlet valve seat (27), preferably centered by the spring (33), and in that the outlet valve body (29) is an annular disk loaded by a spring (31) towards the outlet valve seat (28) and guided movably on the valve control member (18), preferably with a sliding ring seal (30) arranged between the valve control member (18) and the annular disk.
 18. The differential piston pump as claimed in claim 17, wherein the annular disk and the disk comprise conical or spherical seat surfaces, and, preferably, the valve seats (27, 28) are of conical or spherical construction.
 19. The differential piston pump as claimed in claim 15, wherein the distance, when viewed in the longitudinal direction of the valve control member (18), between the shoulders (23, 24) is smaller than the distance between the valve bodies positioned on the valve seats (27, 28), preferably in such a way that, at the top and bottom dead centers of the differential piston (K) assigned to the outlet and inlet valve arrangement (A), both valve bodies are seated and play is present relative to the respective shoulder (23, 24), preferably in each case of approx. 0.1 to 0.4 mm.
 20. The differential piston pump as claimed in claim 15, wherein the valve control member (18) extends between the outlet valve seat (28) and the drive shaft (12) and between the valve seats (27, 28) through guides (21, 37) fixed to the housing.
 21. The differential piston pump as claimed in claim 14, wherein the valve seats (27, 28) and the one guide (21) are arranged in sleeve members (25, 26), which are mounted in a housing chamber between a housing wall (10) adjacent to the drive shaft (12) and a housing cover (9) and comprise an inlet (5) and two outlets (34, 6), and in that a further inlet is arranged in the housing wall (10).
 22. The differential piston pump as claimed in claim 1, wherein the eccentric (13) for the differential piston (K) and the eccentric (14) for the valve control member are constructed with at least substantially identical dimensions and with identical eccentricities (C).
 23. The differential piston pump as claimed in claim 1, wherein the extent of the eccentricity (C) of the eccentric (13, 14) is adjustable and/or the eccentric (13, 14) is arranged replaceably on the drive shaft. 